Electric discharge device



March 21, 1961 G. H. REILING ETAL 2,976,451

ELECTRIC DISCHARGE DEVICE Filed June s, 1959 2 Sheets-Sheet 1 Cs Rb K NaLi ALKAL/ ME TAL-MERCUR) POOL EXCITATION POTENTIALS INVENTORSZ GILBERTH. REILING, ANDREW O. JENSEN BY/P THEIR ATTORNEY.

March 21, 1961 Filed June 8, 1959 ARC DROP IN VOLTS 0 Q G. H. REILINGETAL 2,976,451

ELECTRIC DISCHARGE DEVICE 2 Sheets-Sheet 2 FIG.3.

WHOLLY MERCURY POOL ALKALI METAL-MERCURY POOL TUBE I I I I 60 I00 I40I80 WALL TEMPERATURE IN DEGREES CENTIGRADE ARC DROP IN VOLTS ALKALIMETAL-MERCURY POOL TUBE I l I I I 40 80 I20 I60 ANODE CURRENT IN AMPERESINVENTORSZ GILBERT H. REILING, ANDREW O. JENSEN,

g m ATTORNEY.

ELECTRIC DISCHARGE DEVICE Gilbert H. Reiiing, Schenectady, and Andrew 0.Jensen,

Guilderland, N.Y., assignors to General Electric Company, a corporationof New York Filed June 8, 1959, Ser. No. 818,874 5 Claims. (Cl. 313-171)-peres has been near the ionization potential of mercury orapproximately volts, with the arc drop increasing in devices adapted forhigher currents. This voltage drop has been considered necessary tomaintain the electron emission from the mercury cathode spot and toreplenish the positive ions which are ordinarily lost in neutralizingspace charge normally occurring in the device.

Although the voltage drop of mercury cathode ignitrons is not generallyconsidered substantially large and presents no substantial problems formany applications, it has been long considered desirable to provide anignitron-type electric discharge device adapted for lower voltage dropthan encountered in ignitrons, thus to obtain higher operatingefliciency and higher operating tempera tures. Higher operatingefiiciency is not only generally desirable in all devices of this typebut would enable wider applications of ignitron-type devices; Whilehigher operating temperatures are desirable in that they would obviatethe need for liquid cooling generally required for mercury pool ignitronoperation and would make feasi ble air cooling or other relatively lowcapacity cooling means in applications Which previously required Watercooling. Additionally, at the higher currents of the mentioned rangethere is a tendency in mercury pool tubes for the arc to run free ormove rapidly and erratically over the surface of the cathode. Forvarious reasons, including the desire to minimize the arc drop andobtain better control of vapor pressure in the device, it is generallydesirable to stabilize the are or anchor the arc spot to a restrictedarea of the pool. Heretofore it has not been possible to obtain thedesired spot anchoring at high currents.

In addition to various redesigns of the anode-cathode geometries of thedevice, the efforts previously made to reduce arc losses have beendirected toward substituting cathode materials having lower ionizationpotentials for the mercury cathode or the confinement of the dischargeby magnetic means. By some of these means reduction in :arc losses havebeen realized; however, other difficulties have rendered these meanscommercially infeasible. For example, some of the ionizable materialssought to be substituted for the mercury have been found 'too expensiveto be employed in any appreciable quantity in a commercially feasibledevice, have been found so :reactive chemically. as to be incompatiblewith known United States Patent 0 2,976,451 Patented Mar. 21,

tube envelope and seal materials and also so reactive as to constitutesubstantial health hazards in normal manufacturing operations andhandling. Further, some materials sought to be substituted for mercuryrequired preheating to enable ionization and initial operation or starting of the device. Devices requiring pre-heating as well as anyrequiring auxiliary equipment, such as magnetic field producing means,have been found generally undesirable for the obvious reason that theyadd considerably to the effort and cost of manufacturing andmaintenance. In connection with the anchoring of the cathode spot it hasbeen found that at high currents, in the order of amperes or more,elements placed in the device for serving as an anchor point have becomedewetted of mercury and thus rendered incapable of serving effectivelyas an anchor. 1

Accordingly, the primary object of the present inven' tion is to providea new and improved cold cathode arc discharge device adapted for low arclosses.

Another object of the present invention is to provide a new and improvedcold cathode 'arc discharge device of the ignitron type adapted forgreater operating eificiency than ignitrons incorporating a whollymercury pool cathode and thus eiiective for adapting ignitron-typedevices for wider applications.

Another object of the present invention is to providea new and improvedcold cathode arc discharge device adaped for affording low arc losses bymeans of a contained ioniza-ble medium and requiring no auxiliaryequipment, such as pre-heating means and magnetic field producing means,for low arc loss operation Another object of the present invention is toprovide a new and improved cold cathode arc discharge deviceincorporating a liquid cathode pool, the major constituent of which cancomprise mercury for afiordin-g, in addition to the desired low arclosses, all of the advantages and minimizing other undesirable effectsof arc instability.

Another object of the present invention is to provide a new and improvedcold cathode arc discharge device of the ignitron type including meansfor suppressing the internal pressure of the device and thus adaptingthe device for higher operating temperatures.

Another object of the present invention is to provide an improved coldcathode arc discharge device adapted for withstanding higher inversevoltages without'arc back and which may be satisfactorily cooled by airor other means not having the substantial cooling capacity ofwater-cooling means.

Still another object of the present invention is topro vide a new andimproved cold cathode arc discharge device which is commerciallyfeasible from the standpoints of function and cost and presents nosubstantial difficulties in the way of health hazards in manufacture orhandling of materials employed therein.

Further objects and advantages of the invention will become apparent asthe following description proceeds and the features of novelty whichcharacterize the invention will be pointed out with particularity in theclaims annexed to and forming part of this specification.

In carrying out the objects of the present invention there is provided acold cathode arc discharge device of the ignitron type including anenvelope having an insulative wall section electrically separating acathode.

However, when operating temperature is attained, excited metastablemercury atoms ionize the alkali metal atoms and the ionized alkali metalneutralizes the effects of the space charge in the device to increasethe current conducting capacity of flue device. Lower arc drop followssince a greater voltage drop across the device for ionizing mercuryatoms to effect neutralization of the space charge is not required.Additionally, lowering of the arc drop results from a lowering of theanode work function by condensation of alkali metal on the anode. Alsodisposed in the tube is a member of high refractory low sputtering ratematerial which protrudes from the surface of the cathode and is adaptedfor being wetted by the mercury for serving as an anchor for the cathodespot. In order to avoid de-wetting of the member at high currents thealkali metal serves to maintain the member substantially oxide free.

For a better understanding of the invention reference may be had to theaccompanying drawing in which:

Figure l is a sectional view of an electric discharge deviceincorporating a form of the present invention;

Figure 2 is a chart illustrating the excitation potentials of cathodematerials employable in the structure of Figure 1;

Figure 3 is a comparative illustration of the arc-drop versustemperature curves of a wholly mercury pool tube and a tube constructedaccording to the present invention; and

Figure 4 is a comparative illustration of the arc-drop versus current ofa wholly mercury pool tube and a tube constructed according to thepresent invention.

Referring to the drawing, there is shown in Figure 1 a cold cathode arcdischarge device embodying a form of the present invention. The devicecomprises an envelope 1 including an insulative bulb Z constituting aninsulative wall section of the device and which can be advantageouslyformed of glass or ceramic. The upper endof the envelope includes anexternal portion 3 of an anode generally designated 4 which is supportedcentrally in the envelope and includes an enlarged portion bearingatransversely extending planar active surface 5. The anode 4 is formedpreferably of any suitable high refractory metal. Additionally, thementioned external portion 3 can serve effectively as an anodeconnector. A sealing assembly 6 bonded to both the anode and the bulb 2hermetically seals the anode to the bulb and, for reasons which will bebrought out in detail hereinafter, the seal afforded by the assembly 6,as well as all other seals in the device, is selected so as to besubstantially oxide-free at least on the inner surfaces thereof.

Closing the lower end of the bulb 2 to complete the envelope is a metalheader 7 carrying a cathode lead 8. The header 7 can also beadvantageously formed of a high refractory metal and is sealed to thelower end of the bulb by a sealing ring 9 adapted also for presenting asubstantially oxide-free inner surface.

Contained in the bottom of the envelope is a liquidpool-type electrode10 comprising mercury and an alkali metal. The percentage composition ofthe cathode 10 and the function thereof in the operation of the devicewill be brought out in greater detail hereinafter.

Associated with the electrode 10 is a starter electrode generallydesignated 11 which, as seen in Figure 1, can comprise a dielectricstarter including a dielectric element 12 partially immersed in thecathode and supported by a conductive lead 13 extending through the sideWall of the bulb 2. The lead 13 is also sealed in the device by asealing ring 14 adapted for presenting a substantially oxide-free innersurface. The starter 11 can be one of various generally availabledielectric starters adapted for pool tube firing. Alternatively, a bandstarter, or one in which an electrode surrounds the external wall of theenvelope, or a resistance ignitor, both of which types are widely known,may be employed in place of the dielectric starter 11.

Secured on the inner surface of the header 7 and protruding centrallyfrom the surface of the cathode 10 is a cathode spot anchoring membr 15.The member 15 is formed of a metal which is high-refractory in natureand has a low sputtering rate such, for example, as molybdenum,zirconium, tungsten, vanadium, niobium, and columbium. Of thesematerials, molybdenum has been found particularly highly satisfactory asa spot anchoring member.

As pointed out above, the cathode 10 comprises mercury and a quantity ofalkali metal. The alkali metal is preferably selected from the group ofalkali metals having first ionization potentials below the first excitedmetastable level 4.7 e.v. of mercury. As seen in Figure 2, cesium,rubidium, and potassium constitute alkali metals having ionizationpotentials below the first excited metastable level 4.7 e.v. of mercury.The alkali metals sodium and lithium, as seen in Figure 2, have firstionization potentials below the excited metastable level 5.4 e.v. ofmercury. These alkali metals are employable in the cathode 10. However,sodium and lithium have relatively low vapor pressures and in somedevices the use of rubidium, potassium and cesium appears preferable.The advantages of employing a pool cathode including both mercury and analkali metal and the operational relationships of the ionizationpotentials of the alkali metals and the excited metastable levels ofmercury will now be brought out in detail.

During initial operation of the present device, it functions in a mannersimilar to that of the conventional ignitron incorporating a whollymercury pool cathode. That is, the starter 1 1 initiates an arc to thesurface of the pool to establish a cathode spot and the electronstraveling from the cathode toward the anode surface effect ionizationand metastable excitation of mercury molecules in the region between thecathode and anode, thus to supplement the current flow toward the anodeby providing positive ions effective for neutralizing the space chargewhich normally results in the device. The space charge neutralizationhas the desirable effect of increasing the electron flow or currentcarrying capacity of the device. However, in the conventional tube thevoltage drop across the device in the low current range of approximately10 to amperes is near the ionization potential of the mercury, thisvoltage drop increasing as the current is increased. This voltage dropis required to maintain the electron emission from the cathode spot andto replenish the positive ions lost from the discharge.

In the presently disclosed device and, as pointed out above, theoperation is initially the same as in the conventional wholly mercurycathode arc device. However, soon after starting the long-livedmetastable mercury atoms will have become excited at least to themetastable level of 4.7 e.v. and if either rubidium, potassium or cesiumare used will effect ionization of the alkali metal atoms and thecathode 10 by means of what is currently understood to be a two-stagecollision process, 'If sodium or lithium are used the ionization by themetastable mercury atoms will occur when the latter have become excitedto the 5.4 e.v. level. The thus ionized alkali metal has the desirableeffect of neutralizing the space charge and results in increased efficiencypwhich adapts the device for applications where devices of lessefficiency would be unacceptable.

Additionally, the work function of the cathode pool is not believed tobe afiected by the presence therein of the alkali metal. However, inoperation the alkali metal in the device is believed to be condensed onthe active surface 5 of the anode at least to the extent of a monolayerwhich is understood to have a lowering elfect on the work function ofthe anode. These factors are also believed effective in decreasing thevoltage drop across the device.

Further advantage of the presence of the alkali metal in the cathodematerial is believed to result from the depressing effect thereof on thevapor pressure of the mercury. With the vapor pressure thus depressedthe device is better adapted for withstanding inverse voltages and lessreliance is placed on cooling of the device to require satisfactoryoperation. That is, in wholly mercury pool tubes substantial cooling,such as by a circulating liquid coolant, is required to insure a vaporpressure in the device at which the tube would operate as eflectively aspossible without substantial danger of undesirable arc back. The presentdevice can operate cooled, for example, only by circulating air andstill be elfective for withstanding substantial inverse currents oravoiding arc back.

The alkali metal also has the desirable effect of maintaining thesurface of the spot anchoring post 15 substantially clean or oxide-free.In prior devices, when current approached, for example, 100 amperesanchoring elements have become oxidized and'de-wetted of mercury causingthe meniscus of the mercury to be convex or to turn down at the point ofjuncture with the post 15-; and, thus, at high currents the post wasrendered less effective for anchoring a spot. In the present inventionthe alkali metal maintains the post 15 clean or substantially oxidefree, thus to cause the meniscus to be concave in the manner illustratedin Figure 1 or, in other words, to avoid any substantial de-wetting ofthe member 15 and thus to insure elfectiveness of the member 15 inanchoring the cathode spot at high currents as well as relatively lowcurrents. In addition to attributing to the low arc drop, the anchoringof the spot has the deisrable effect, by avoiding free running of theare over the surface of the pool, of reducing evaporation and vaporblast, known as the Tanberg efiect, from the cathode spot. Additionally,better control of the vapor pressures is possible, resulting in abilityof the device to withstand higher applied inverse voltages and avoidanceof the need for de-ionization surfaces such as grids and bafiles.

Still further, the presence of alkali metal with mercury in theatmosphere of the device does not cause undesirable low electricalresistance across the internal insulative surface of the envelope, aswould be expected if only alkali metals were employed. This advantagedesirably obviates the need for external heaters and other means toprevent unwanted electrical conduction across the envelope wall.

The above-described functional advantages can be attained oversubstantially wide percentage composition ranges of mercury and alkalimetal. For example, decreases in arc drop have been obtained withmixtures of mercury and rubidium where the rubidium has constituted upto about 80% of the mixture. However, in constructing a commerciallyfeasible device it is deemed desirable to utilize as much mercury aspossible relative to the amount of alkali metal employed because of therelative ease of initiating an arc to mercury and the ability of mercuryreadily to supply a copious quantity of electrons for high currentpurposes. Additionally, mercury is more readily available and lessexpensive than the alkali metals; and the alkali metals, because oftheir high chemical reactivity, can, when used in substantial amounts,present substantial problems in connection, for example, with handlingand the provision of compatible seals. Further, it has been found thatmixtures containing substantial quantities of alkali metal relative tomercury have a tendency to solidify which precludes starting of a devicewithout first pre-heating. While it is recognized that the presentlydisclosed device can be supplied with PIC-heating means if for anyreason it is desirable to provide a device viously known to beobtainable with the use'of a wholly mercury pool cathode and alsowithout introducing prob lems or requiring auxiliary equipment notheretofore required with wholly mercury pool cathodes. These desiderataare obtainable when the pool 10 comprises about from .02% to about 20%of an alkali metal and the remainder being mercury. Alkali metalsselected from the group including cesium, rubidium and potassium andcombinations thereof, including alloys and mixtures are consideredparticularly advantageous. These particular alkali metals, as seen inFigure 2, have ionization potentials lying below the excited metastablelevels 4.7 e.v. and 5.4 e.v. of the mercury and, thus, are readilyionizable by the metastable atoms of the mercury to provide theabove-described ionization effect for neutralizign the space charge inthe device, The other alkali metals sodium and lithium are employable insome applications and, as seen in Figure 2,' have ionization potentialsbelow the excited metastablelevel, 5.4 e.v. of the mercury and are alsoionizable by the metastable atoms of the mercury to provide thementioned ionization effect for neutralizing the space charge.Illustrated, for example, in Figure 3 and indicated. by the solid lineis an arc-drop versus temperature .curve of a device constructedaccording to the present invention and including a liquid cathode poolcomprising 1.8% alkali metal, which in this particular case' wasrubidium, and the remainder mercury. As shown ,by

this curve an arc drop of approximately only 5.4'volts was attained at atemperature extending over a substantially wide high temperature range.tive purposes the dash line'is provided in Figure'3'to illustrate thefact that in wholly mercury pool tubes the lowest arc drop issubstantially higher than the presently considered type of device and isattainable only when the tube is operating at a substantially lowertem-' perature than that at which the lowest arc drop is experienced inthe presently disclosed type of device.

Additionally, in Figure 4 the curves illustrate arc-.

drop versus anode current of the same devices. The

dash line represents the operation of the wholly mercury pool deviceand, as illustrated, the current of this device appears to changerelatively sharply which is generally considered characteristic of thistype of device. 'For, example, at 40 amperes the arc drop of this deviceis approximately 11 volts compared with 5.6 volts for the device of thepresent invention, the curve of which is illustrated by a solid line.Comparable low arc drops are obtainable when comparably small amounts ofalkali metals or combinations of such metals are included in 1 a cathodepool, the remainder of which is mercury.

Additionally, it has been found that mixtures of the allower than thoseattainable in wholly mercury cathode 7 devices. Additionally, the alkalimetal serves to de press the mercury vapor pressure to adapt the deviceFor comparai for withstanding substantially greater inverse voltages andhigher operating temperatures to enable cooling, for example, by air.Further, the alkali metal present in the tube atmosphere does notsignificantly decrease the electrical resistance of the insulative wallsurfaces between the opposed cathode and anode. Still further, in suchsmall amounts use of the alkali metal does not become prohibitivelyexpensive or present any substantial problems regarding handling and theprovision of compatible seals and envelope materials. When more thanabout alkali metal is employed, the arc tends to anchor on the junctureof the glass wall and pool edge which has the undesirable effect ofrendering the are less stable and damaging the envelope. Additionally,higher amounts of alkali metal have a solidifying effect on the pool andthus necessitate pre-heating means to enable starting. Where a solidpool or the use of heating means are not objectionable, the higheramounts of alkali metal are employable for obtaining the variousadvantages described above.

While a specific embodiment of the invention has been shown anddescribed, it is not desired that the invention be limited to theparticular form shown and described, and it is intended by the appendedclaims to cover all modifications within the spirit and scope of theinvention.

What is claimed as new and desired to secure by Letters Patent of theUnited States is:

1. A cold cathode arc discharge device comprising an envelope, an anodein said envelope, a liquid cathode in said envelope in spaced insulatedrelation with respect to said anode, said cathode consisting of alkalimetal in an amount between about .02% to about 20% and the remaindermercury, and means for initiating an arc discharge between said anodeand cathode.

2. A cold cathode arc discharge device comprising an envelope, an anodein said envelope, a cathode in said envelope, said cathode having anactive portion consisting of mercury and an alkali metal, a highrefractory metal member protruding from the surface of said cathodeadapted for being wetted by said mercury, thereby to anchor a cathodespot in said device, said alkali metal being effective for minimizingde-wetting of said member during operation of said device, thus toinsure continued anchoring of said spot, and means for initiat- 8 ing anarc discharge between said anode and cathode.

3. A cold cathode arc discharge device comprising an envelope, an anodein said envelope, a cathode in said envelope, said cathode having anactive portion c0nsisting of a pool of mercury and alkali metal withsaid alkali metal consisting from about 02% to about of said activeportion of said cathode, a post of high refractory low sputter ratemetal protruding from the surface of said pool and adapted for beingwetted by said mercury thereby to anchor a cathode spot in said device,said alkali metal being effective for maintaining said postsubstantially oxide-free, thereby to minimize de-wetting of said postduring operation of said device, and means for initiating an arcdischarge between said anode and cathode.

4. A cold cathode arc discharge device according to claim 2, whereinsaid envelope includes an insulative wall section separating said anodeand cathode, said post comprises molybdenum, said cathode having anactive portion consisting of between about .02% to about 20% alkalimetal and the remainder mercury, and said alkali metal is additionallyeifective for being ionized to neutralize any space charge in saiddevice and to lower the work function of said anode, thereby to increasethe current capacity of said device.

5. A cold cathode arc discharge device comprising an envelope, an anodein said envelope, a cathode in said envelope in spaced insulatedrelation with respect to said anode, said cathode consisting of at leastone alkali metal selected from the group consisting of cesium, rubidiumand potassium in an amount between about 02% to about 20% and theremainder mercury, and means for initiating an arc discharge betweensaid anode and cathode.

References Cited in the file of this patent UNITED STATES PATENTS1,757,605 Ulrey May 6, 1930 2,152,201 Miles Mar. 28, 1939 2,500,153 Corket al Mar. 14, 1950 2,541,842 Teare Feb. 13, 1951 2,650,319 MiddletonAug. 25, 1953 2,651,737 Marshall Sept. 8, 1953

